1. Field of Industrial Application
The present invention relates to a plasma processing system having an improved plasma source capable of supplying ions, electrons, neutral radicals and ultra-violet and visible light useful for chemical vapor disposition (CVD) or etching processes in the fabrication of devices on semiconductor wafers in semiconductor industry.
2. Discussion of Related Art
Plasma sources are an indispensable tool for the present and future semiconductor wafer processing in semiconductor industry. Three important conditions of such sources are higher plasma density, higher radial uniformity of the plasma over the wafer surface, and large-area plasma. In this respect, conventional high density plasma sources have limited applications owing to their design rules. This matter is explained by reference to a helical resonator plasma having a conventional geometry. A schematic diagram of the conventional helical resonator plasma source producing high density plasma is shown in FIG. 8. Since only the plasma generation process is of interest, a wafer holding stage, a vacuuming port and vacuum components are not shown in FIG. 8. The helical resonator plasma source has a helical coil 51 made of metal and wound around a dielectric tube 52 which is usually made of quartz. One end of the helical coil 51, usually the lower end, is grounded while the other end is open. The length of the helical coil is L1=.lambda./4, where .lambda. is the wavelength of the rf frequency applied to the helical resonator and n is an integer. A metal cylinder 53, usually made of aluminum, is placed around the helical coil 51. The metal cylinder 53, the helical coil 51, and the dielectric tube 52 are placed coaxially on a top plate 59 of a process chamber 55. This top plate 59 is made of metal and has a circular hole with a diameter equal to the diameter of the dielectric tube 52. A process gas is fed through a gas inlet port 58 formed at the upper end of the dielectric tube 52. An rf power generated from an rf power source 57 is fed to a point 54 on the helical coil 51 through a matching circuit 56. The rf power source 57 operates at a constant frequency that lies usually in the range of 1 MHz to 40 MHz. When the length of the helical coil 51 is taken as an integral multiple of a quarter of the wavelength, the composite structure begins to resonate. At this condition, the electromagnetic field within the helical coil 51 can ignite and maintain a plasma in the dielectric tube 52 at lower pressures.
A wafer holder is arranged at the lower end of the process chamber 55. A wafer to be processed is loaded on the wafer holder. The plasma generated within the dielectric tube 52 is mainly diffused toward the wafer in the process chamber 55 through the circular hole of the top plate 59.
First, a major problem concerning the configuration of the above-mentioned high density plasma source making the helical resonator plasma is in the controllability of radial plasma uniformity. The plasma is generated in the small-diameter dielectric tube 52 and then introduced into the large-diameter process chamber 55. Once the high density plasma enters into the process chamber 55, the plasma generated various species, for example, ions, electrons, etc., diffuse radially outward in addition to their flight downstream. This diffusion process yields a nonuniform plasma density across a radial line of the process chamber 55 as shown in FIG. 8. FIG. 8 shows the plasma density distribution characteristic of the plasma diffused in the process chamber 55. The lateral axis means a distance in a radial direction across the process chamber 55 and the longitudinal axis refers to the plasma density. As shown by the plasma density characteristic curved line 60 in FIG. 8, the plasma density is at a high level at the center position while it is at a low level at the outer edges. Thus, the plasma diffused into the process chamber 55 from the dielectric tube 52 becomes nonuniform in the radial direction of the process chamber 55. Even though a large-diameter dielectric tube is used instead of the dielectric tube 52, the above problem can not be solved. Thus, without additional hardware, for example, magnetic multipole confinement, a radially uniform plasma can not be obtained with the above helical resonator plasma source.
Secondly, the open end of the helical coil 51 has a higher voltage when the helical coil 51 begins to resonate. This higher voltage causes generation of a capacitively coupled plasma that in turn results in sputtering the wall of the dielectric tube 52, which is close to the open end of helical coil 51. This causes a contamination of the plasma.
Because of these qualities, the conventional helical resonator plasma source has limited application in plasma assisted wafer processing, especially in large-area wafer processing. In order to avoid the above mentioned disadvantages, the configuration of helical resonator plasma source has to be improved.